1. Field of the Invention
The present invention relates to compositions comprising alkyl phenol sulfonic acids and to methods for inhibiting undesired polymerization of aromatic ethylenically unsaturated monomers using alkyl phenol sulfonic acids.
2. Background
Aromatic ethylenically unsaturated monomers, such as styrene (C.sub.6 H.sub.5 CH.dbd.CH.sub.2), are used extensively for the manufacture of plastics. These monomers undergo undesirable thermal and free radical polymerization during storage, shipping, and particularly during processing. Such polymerization can cause fouling of distillation columns and other equipment used for processing the monomers and can render the monomers unfit for use without further treatment. To minimize polymerization, it is common practice to add to the monomer recovery stream, compounds which have polymerization inhibiting activity.
A wide variety of compounds have been used as polymerization inhibitors. U.S. Pat. No. 4,654,450, issued to Miller, discloses use of mixtures of dialkylhydroxylamines and alkyl benzene sulfonic acids as polymerization inhibitors. U.S. Pat. No. 4,425,223, issued to Miller, discloses use of mixtures of alkyl benzene sulfonic acids and alkyl esters of phosphonic acid to protect hydrocarbon processing equipment against fouling. U.S. Pat. Nos. 4,061,545 and 4,177,110, issued to Watson, disclose use of a mixture of tertiary-butylcatechol and phenothiazine as a polymerization inhibitor for vinyl aromatic compounds. U.S. Pat. No. 3,390,198, issued to Leston, discloses use of mono and dialkylcatechols as polymerization inhibitors for hot styrene.
Styrene manufacturers go to great lengths to remove air from the product recovery section of their plants. Thus, a polymerization inhibitor must work in the absence of oxygen. Current industry polymerization inhibitors include 2,4- and 2,6-dinitrophenol, plus alkylated homologues such as 2,4-dinitro-o-cresol and 2,4-dinitro-sec-butylphenol.
All of these products suffer from a common problem, toxicity. For example, LD.sub.50 for 2,4-dinitrophenol is 30 mg/kg when administered to the interperitoneal cavity of a rat. In addition, both the dinitrophenols and dinitro-o-cresol have very low solubility, less than 5%, in either styrene or its precursor ethylbenzene. Companies that use either of these two products typically make up solutions in hot styrene or ethylbenzene. This provides increased solubility, however, the companies are then dealing with a known toxin dissolved in a hot carcinogen. Solubility problems can be overcome by using products such as dinitro-sec-butylphenol. The alkyl group does not add any activity to the product, however, so, while solubility in hydrocarbons is increased, product activity in terms of activity per pound is decreased.
It has now been discovered that alkyl phenol sulfonic acids provide outstanding polymerization inhibiting activity for aromatic ethylenically unsaturated monomers.